One of the single most important concepts in science

This is REALLY easy to build. SUPER cool to watch (though most adults can’t figure out how it works until you tell them). And it teaches one of the MOST IMPORTANT concepts there is in science. Guaranteed to keep small kids and cats busy for hours :-)

Here’s what you need:

  • can with a lid
  • small heavy rock or large nut (about 1″ in diameter)
  • two paper clips
  • rubber band (at least 3″ x 1/4″)

You’ll need two holes punched through your container – one in the lid and the bottom. Thread your rubber band through the heavy washer and tie it off (this is important!). Poke the ends of the rubber band through one of the holes and catch it on the other side with a paper clip. (Just push a paper clip partway through so the rubber band doesn’t slip back through the hole.) Do this for both sides, and make sure that your rubber band is a pulled mildly-tight inside the can. You want the rock (I used a hexnut in the video) to dangle in the center of the can without touching the sides of the container.

Now for the fun part… gently roll the can on a smooth floor away from you. The can should roll, slow down, stop, and return to you! If it doesn’t, check the rubber band tightness inside the can.

The large hex nut or small rock is a weight that twists up the rubber band as the can rolls around it. The kinetic energy (the rolling motion of the can) transforms into potential (elastic) energy stored in the rubber band the free side twists around. The can stops (this is the point of highest potential energy) and returns to you (potential energy is being transformed into kinetic). The farther the toy is rolled the more elastic potential energy it stores.

This is one of the most fundamental – and most important – concepts in science: energy cannot be created or destroyed, but it can change forms. This is technically known as Conservation of Energy, or The First Law of Thermodynamics.

In our case, it moves from kinetic to potential and back again, over and over. With each roll, the can moves less and less because there’s also a small percentage of the energy being transformed into heat through friction with the table and air resistance. In space, this can would go on forever if you could keep the rubber band from freezing. :)


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